The VAV terminal unit is fitted with two differential pressure sensors for measuring the volume flow rates, one in the cold air flow and one in the total air flow.

The control components (attachments) include two differential pressure transducers that transform the differential pressure (effective pressure) into an electric signal, two controllers, and two actuators; the control functions can be achieved with a Compact controller or with individual components.

In most cases, the setpoint value for the dual duct terminal unit comes from a room temperature controller.

The room temperature controller 'leads' the cold air volume flow controller and alters the setpoint for the cold air flow rate between 0 and the maximum volume flow Vmax. The controller compares the actual value with the setpoint value and alters the control signal of the damper actuator if there is a difference between the two values.

The warm/total air controller is set to the minimum volume flow rate Vmin and controls the warm air damper blade. As a consequence, a corresponding proportion of warm air is added. As the demand for cooling increases, the warm air damper blade closes such that eventually only cold air flows.

An integral attenuator reduces the noise that is created by the restriction of the airflow.

The airflow velocity at the room end is, due to the larger rectangular cross section, about half the velocity in the circular duct.

Volume flow rate ranges

The minimum differential pressure of VAV terminal units is an important factor in designing the ductwork and in rating the fan including speed control.

Sufficient duct pressure must be ensured for all operating conditions and for all control units. The measurement points for fan speed control must be selected accordingly.

The volume flow rates given for VAV terminal units depend on the nominal size and on the control component (attachment) that is installed. The table gives the minimum and maximum values for a VAV terminal unit. Some control components may only have a limited volume flow rate range. This applies in particular to control components with a static differential pressure transducer. For volume flow rate ranges for all control components refer to our Easy Product Finder design programme.

Nominal size

V
l/s

V
m³/h

①
∆pst min
Pa

②
∆pst min
Pa

∆V
± %

∆Vwarm

125

45

162

120

160

8

17

125

60

216

120

160

7

15

125

100

360

120

160

5

12

125

150

540

120

160

5

7

160

75

270

120

140

8

17

160

100

360

120

140

7

15

160

170

612

120

140

5

12

160

250

900

120

140

5

7

200

120

432

120

140

8

17

200

180

648

120

140

7

15

200

280

1008

120

140

5

12

200

405

1458

120

140

5

7

250

185

666

120

145

8

17

250

270

972

120

145

7

15

250

470

1692

120

145

5

12

250

615

2214

120

145

5

7

315

310

1116

120

160

8

17

315

420

1512

120

160

7

15

315

720

2592

120

160

5

12

315

1030

3708

120

160

5

7

400

505

1818

120

160

8

17

400

710

2556

120

160

7

15

400

1250

4500

120

160

5

12

400

1680

6048

120

160

5

7

Nominal sizes

125 – 400 mm

Volume flow rate range

45 – 1680 l/s or 162 – 6048 m³/h

Volume flow rate control range

Approx. 30 to 100% of the nominal volume flow rate

Minimum differential pressure

120 Pa

Maximum differential pressure

1000 Pa

Operating temperature

10 – 50 °C

Quick sizing tables provide a good overview of the room sound pressure levels that can be expected. Approximate intermediate values can be interpolated. Precise intermediate values and spectral data can be calculated with our Easy Product Finder design programme.

The first selection criteria for the nominal size are the actual volume flow rates Vmin and Vmax. The quick sizing tables are based on generally accepted attenuation levels. If the sound pressure level exceeds the required level, a larger air terminal unit and/or a silencer is required.

Connecting spigots for warm and cold air arranged at an angle of 90°. Up to nominal size 200 an angle of 60° is also possible, hence ideal for for the refurbishment of older systems with dual duct units.

High control accuracy (even with upstream bend R = 1D).

Ready-to-commission unit which consists of the mechanical parts and the electronic control components. Each unit contains two averaging differential pressure sensors for volume flow rate measurement, one in the cold air flow and one in the total air flow, two damper blades, and an integral attenuator. Factory-assembled control components complete with wiring and tubing.

Installation details, Basic information and nomenclature

Installation details

Basic information and nomenclature

Bend

A bend with a centre line curvature radius of at least 1D – without an additional straight duct section upstream of the VAV terminal unit – has only a negligible effect on the volume flow rate accuracy.

Installation and commissioning

Any installation orientation

Connecting spigots for warm and cold air arranged at an angle of 60° (TVM-S) or 90° (TVM)

Return edges of the casing with drilled holes suitable for threaded rods

Upstream conditions

The volume flow rate accuracy ΔV applies to a straight upstream section of the duct. Bends, junctions or a narrowing or widening of the duct cause turbulence that may affect measurement. Duct connections, e.g. branches off the main duct, must comply with EN 1505. Some installation situations require straight duct sections upstream.

Space required for commissioning and maintenance

Sufficient space must be kept clear near any attachments to allow for commissioning and maintenance. It may be necessary to provide sufficiently sized inspection access openings.

Definition of noise

① Air-regenerated noise
② Case-radiated noise

Principal dimensions

ØD [mm]

VAV terminal units made of stainless steel: Outside diameter of the spigot

VAV terminal units made of plastic: Inside diameter of the connecting spigot

The static minimum differential pressure is equal to the pressure loss of the VAV terminal unit when the damper blade is open, caused by flow resistance (sensor tubes, damper mechanism)

If the pressure on the VAV terminal unit is too low, the setpoint volume flow rate may not be achieved, not even when the damper blade is open

Important factor in designing the ductwork and in rating the fan including speed control

Sufficient duct pressure must be ensured for all operating conditions and for all terminal units, and the measurement point or points for speed control must have been selected accordingly to achieve this

Construction

Galvanised sheet steel

Casing made of galvanised sheet steel

Parts in contact with the airflow as described for the product type

External parts, e.g. mounting brackets or covers, are usually made of galvanised sheet steel

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